1. Field of the Invention
The present invention relates to image pickup apparatuses, and, more particularly, to an image pickup apparatus capable of reducing random noise and a reading method of the image pickup apparatus.
2. Description of the Related Art
In current years, image pickup apparatuses having a total number of pixels exceeding ten million pixels are used for digital still cameras for mainly capturing still images, and image pickup apparatuses having several million pixels are used for movie cameras for mainly capturing moving images. As such an image pickup apparatus, a photoelectric conversion apparatus is used.
In such an image pickup apparatus, the increase in the number of pixels is achieved by the reduction in the size of each pixel. As a result, however, the opening area of each pixel is reduced. This leads to low sensitivity and optical shot noise. Furthermore, with the decrease in the amount of electric charge processed by pixels, noise caused by an image pickup apparatus has become more pronounced.
The noise reduction configurations of amplifying image pickup apparatuses having an amplification unit in each pixel are known. With such a noise reduction configuration, it is possible to reduce fixed pattern noise caused by an amplification unit in each pixel to an allowable image level. With the reduction in the size of each pixel, the size of a MOS transistor functioning as an amplification unit in the pixel is also reduced and the problem of 1/f noise, which is a kind of random noise, arises. The 1/f noise increases in inversely proportional to the size of a MOS transistor.
Currently, in amplifying image pickup apparatuses having a smaller pixel size, new noise occurs. That is, when the relationship between a channel length L of a MOS transistor and a channel width W of the MOS transistor satisfies LW<1 μm2, noise called Random Telegraph Signal (RTS), which is a kind of random noise, becomes predominant. This leads to the reduction in image quality. A specific pixel having two or more valued noise appears on a screen at a certain probability as the RTS noise. When such a specific pixel is observed for a predetermined period at a low illumination level, it looks as if the flicker of a pixel occurs. Accordingly, the RTS noise is also called flicker noise.
Japanese Patent Laid-Open No. 2005-210701 discloses a random noise reduction configuration. In a disclosed random noise reduction method using a circuit, the reduction in noise caused by a pixel amplification unit is achieved by reading each of noise and a signal from the same pixel a plurality of times and averaging them.
In order to effectively reduce random noise, since noise is reduced by 1/√n (n denotes the number of reading times), it is desirable that the number of reading times be increased. In the related art, however, in order to increase the number of reading times, it is necessary to increase the number of averaging memories in accordance with the number of reading times. This leads to the increase in a chip area.